1. Field of the Invention
The present invention relates to an automated precision liquid metering apparatus, and more particularly to an automated precision liquid metering apparatus which includes a plurality of injectors that are used as metering devices and can be controlled through a microcomputer and a control circuit to suck a fixed amount of specified liquid from a storage container and then be shifted to release the sucked liquid into a receptacle on an electronic balance at where the specified liquid is precisely measured.
2. Description of the Prior Art
Most of the currently commercially available automatic liquid metering apparatuses use metering valves to precisely control the amount of a specified liquid allowed to flow from a storage container to a receptacle. FIG. 1 illustrates a most commonly adopted automatic liquid metering apparatus in which a plurality of storage containers 10 are included, a conveyance tube 11 is provided to extend between each storage container 10 and a receptacle 16 positioned on an electronic balance 15, and a metering valve 12 is connected to the conveyance tube 11 at a point close to the storage container 10. To use the above-described liquid metering apparatus to supply a precisely metered amount of a specified chemical liquid, a signal for opening the metering valve 12 that controls the flow of the specified liquid is sent by a control circuit 14 via a microcomputer 13 to the metering valve 12. Under the siphonic action, the specified liquid flows from the storage container 10 via the conveyance tube 11 into the receptacle 16 on the electronic balance 15. A signal representing a measured weight of the specified liquid in the receptacle 16 is then sent by the electronic balance 15 to the microcomputer 13 and the control circuit 14. When the specified liquid reaches a predetermined amount, the microcomputer 13 and the control circuit 14 sends a signal to close the metering valve 12 and thereby stop the supply of the specified liquid from the storage container 10. In a practical application of the above-described automatic precision liquid metering apparatus, as shown in FIG. 2, there are usually several decades or even several hundreds of storage containers 10, conveyance tubes 11 and metering valves 12 included in the apparatus for the same to work economically and practically.
The following are some disadvantages of the above-described conventional precision liquid metering apparatus of FIG. 2:
1. Each of the storage containers 10 must be equipped with one metering valve 12 to meter the amount of liquid flown out from the storage container 10, and each metering valve 12 requires complicate wiring for forming its control circuit. The manufacturing cost of the whole precision liquid metering apparatus of FIG. 2 is therefore very high.
2. In the event the metering valve 12 each includes a one-stage ON/OFF opening, such opening must be small for the metering valve 12 to precisely meter the liquid flowing therethrough. In this case, the flow rate of the liquid from the storage container 10 to the receptacle 16 is inevitably low and results in an extended time required to complete the supply of the specified liquid.
3. On the contrary, in the event the metering valve 12 each includes a multi-stage opening, although the specified liquid can be quickly supplied and precisely metered, the metering valve 12 would have more complicate structure and the apparatus of FIG. 2 would therefore require higher manufacturing cost.
4. Each storage container 10 requires one conveyance tube 11 that also forms a considerable cost of the whole apparatus of FIG. 2. And, it takes time for the specified liquid to flow from the storage container 10 via the conveyance tube 11 to the receptacle16. Moreover, chemicals in the specified liquid in the conveyance tube 11 tend to deposit because there is no way to stir the liquid and therefore well mix the chemicals in the conveyance tube 11. Under this condition, even the liquid supplied to the receptacle is accurate in its total amount, it does not mean the chemicals in the liquid quantitatively meet specified amounts.
FIG. 3 illustrates another type of automatic precision liquid metering apparatus currently available in the market. In this second conventional precision liquid metering apparatus, the conveyance tubes 11 are omitted. The apparatus of FIG. 3 mainly includes a central shaft 20 that horizontally extends through a rotary table 21 to bring the latter to rotate clockwise or counterclockwise. A plurality of storage containers 22 are mounted along an outer periphery of the rotary table 21. Each storage container 22 is sealed with a cap 23. On each cap 23 there is provided a passage 24 via which liquid flows out from the storage container 22 and an amount of compressed air is supplied into the storage container 22, and a first solenoid valve 25 for controlling the open or close of the passage 24. Insulated connectors extended from positive and negative electrodes of the first solenoid valve 25 are adapted to contact with external positive and negative electrodes to make the first solenoid valve 25. The apparatus of FIG. 3 is also controlled through a microcomputer 26 and a control circuit 27 that sends instructions about the types and amounts of liquid to be supplied from the storage containers 22. When an instruction is sent, the rotary table 21 rotates to bring a storage container 22 having the specified liquid stored therein, that is referred to hereinafter as the selected storage container, to a topmost position on the rotary table 21, so that the cap 23 of the selected storage container 22 faces straight upward for the passage 24 thereof to pivotally connect to an air outlet 28 of an air supply tube 29. A second solenoid valve 34 is used to control the supply of air into the selected storage container 22 via the air supply tube 29. Thereafter, the selected storage container 22 is brought by the rotary table 21 to a lowest position on the rotary table 21, so that the cap 23 of the selected storage container 22 and the passage 24 thereof face straight downward. An electronic balance 30 carried on a traveling cart 31 is adapted to locate below the rotary table 21. The traveling cart 31 is provided with positive and negative electrodes 32 that are adapted to electrically connect to the positive and the negative electrode connectors of the first solenoid valve 25 on the cap 23 of the selected storage container 22, so that the passage 24 on the same cap 23 is opened by the first solenoid valve 25 thereof to allow liquid in the selected storage container 22 to flow into the receptacle 33 on the electronic balance 30. The electronic balance 30 is able to feed back a signal representing the measured weight of the liquid in the receptacle 33 to the control circuit 27 and the microcomputer 26. When the liquid in the receptacle 33 reaches a predetermined amount, the first solenoid valve 25 is shut off and the supply of liquid from the selected storage container 22 into the receptacle 33 is stopped. As in the first conventional precision liquid metering apparatus of FIG. 2, the apparatus of FIG. 3 must include several decades or even several hundreds of storage containers 22 and each of which requires a solenoid valve 25.
The following are some disadvantages of the second conventional precision liquid metering apparatus of FIG. 3 without the conveyance tubes 11:
1. The first solenoid valves 25 have limited usable life that is further shortened by frequent open and close of the first solenoid valves 25.
2. Each storage container 22 has only one passage 24 and it is necessary to supply compressed air into the storage container 22 for an internal pressure thereof to be larger than an external atmospheric pressure to let the liquid out of the storage container 22.
3. When the selected storage container 22 containing compressed air is moved to the lowest position on the rotary table 21, the internal air pressure of the selected storage container 22 tends to cause the liquid to flow out the storage container 22 into the receptacle 33 at unstable speed or even jet into the receptacle 33 and therefore adversely affects the accurate measurement of the liquid.
4. The internal pressure of the selected storage container 22 decreases while the liquid flows out of the storage container 22. When the internal pressure of the selected storage container 22 becomes smaller than the external atmospheric pressure, it would be necessary to charge compressed air into the selected storage container 22 again to empty the remaining liquid in the selected storage container 22.